Polyolefin porous films are widely used as a separator in batteries, capacitors, and the like because the polyolefin porous films show high electrical insulation properties and ionic permeability. Particularly in recent years, lithium ion secondary batteries of high power density and high capacity density are used as the power supply for portable devices as the functions of the portable devices are increased and the weight thereof is reduced. The polyolefin porous film is mainly used also for a separator for batteries for such portable devices.
The lithium ion secondary battery has high power density and capacity density. Since an organic solvent is used for an electrolyte solution, however, the electrolyte solution is decomposed by generation of heat accompanied by abnormal events such as a short circuit and overcharge. This may lead to ignition in the worst case. In order to prevent such events, some safety functions are incorporated into the lithium ion secondary battery, and one of them is shutdown function of the separator. The shutdown function is a function that micro pores of the separator are blocked by thermofusion or the like to suppress ion conduction in the electrolyte solution and to stop progression of an electrochemical reaction when the battery has the abnormal heat generated. Usually, it is supposed that safety is higher as the shutdown temperature is lower. The proper shutdown temperature of a separator comprising polyethylene is one of the reasons that polyethylene is used as a component for the separator. A problem of a battery having high energy, however, is that the temperature within the battery continues increasing even if progression of the electrochemical reaction is stopped by shutdown; as a result, the separator is thermally shrunk and broken, causing a short circuit (short) in two electrodes.
In order to produce a battery with high safety, Patent Literature 1 describes a technology for laminating a heat-resistant layer on a first porous layer containing a thermoplastic resin as a main component to form a separator.